A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
At present, several trends may be observed in lithography. Firstly, increasing throughput requirements of the lithographic apparatus are observed to allow the lithographic apparatus to process e.g. a larger quantity of wafers within a certain time period.
Secondly, accuracy requirements tend to be increased, which, in combination with an enhanced capability to project more detailed, i.e. finer patterns on to the substrate, allows manufacturing of substrates having more detailed patterns. As a consequence of the increasing throughput, moving parts in the lithographic apparatus tend to be moving at higher speed and with higher accelerations. Thereby, disturbances caused by such movements such as acoustic vibrations, tend to increase. Secondly because of increased accuracy requirements, a sensitivity of parts of the lithographic apparatus to such acoustic vibrations, tends to increase, at least relative to the imaging accuracy. Further more, in a lithographic apparatus, many other sources of acoustic vibration may be present, e.g. moving elements of the lithographic apparatus, gas ducts to provide a clean or cleaned gas mixture, cooling devices, fans, etc. Acoustic vibrations by any such source, or by any other source, may have an effect on a part of the lithographic apparatus, such as, however not excluded to the projection system, lens element or mirror element of the projection system, position center of a support position measurement system or substrate table position measurement system, alignment system or any other part of the lithographic apparatus.
In this document, the term acoustic vibration may contain any vibration pattern, e.g. sinusoidal, shock wave, burst, pulse, etc, and in any frequency arrange. It is a thus emphasized that the term acoustic vibration is not to be understood as being a limited to sound waves that are audible to the human ear. The acoustic vibration may be in a gas, liquid or solid.